1. Field of the Invention
The present invention relates to: a charged particle beam inspection apparatus used to observe or inspect a sample while emitting a charged particle beam such as an electron beam onto the sample; and an inspection method using a charged particle beam.
2. Description of the Prior Art
In a manufacturing process of a semiconductor device, an electron beam apparatus such as an electron microscope is used to observe a sample or to measure a line width of a pattern or the like. During the observation or measurement of the sample, such an electron beam apparatus scans the sample while emitting an electron beam onto an observation target portion of the sample, and converts the amount of electrons such as secondary electrons into luminance which is then displayed as an image on a display device.
During such observation or measurement of the sample, an electron beam is emitted (irradiated) onto the sample. Here, this emission of the electron beam causes a phenomenon in which a surface of the sample is charged. Specifically, the irradiated surface of the sample is charged positively or negatively depending on the difference between the charge of charged particles incident on the sample and the charge of charged particles emitted therefrom. Additionally, in accordance with the charge potential on the surface of the sample, the emitted secondary electrons may be accelerated or drawn onto the sample. This changes efficiency of emitting secondary electrons. Consequently, a problem arises that results obtained from detecting the amounts of electrons cannot be stable. Moreover, if the charge potential is unchanged, it does not adversely affect anything, however, if the charge potential changes with time, the charge potential on the surface of the sample changes an acceleration condition or a deflection condition of a primary electron beam. This causes a problem that an accurate measurement is not maintained.
To address these problems, various methods to prevent charging on a sample have been proposed.
As a technique related to the above, Japanese Patent Application Publication No. 2003-142019 discloses a method of controlling charging on a sample surface by use of an acceleration voltage at which the yield of the secondary electrons is greater than 1 and an acceleration voltage at which the yield of the secondary electrons is smaller than 1. This method of controlling the charging on the sample surface with use of the acceleration voltages may not ensure the measurement accuracy before and after the changes of the acceleration voltages.
During the sample observation using the electron beam apparatus, the phenomenon of charging the sample occurs as described above. In this respect, if electrical connection of the sample is possible, as in the case of a semiconductor wafer, for example, the charging phenomenon on the sample is less likely to occur, because the wafer can discharge electricity by having a conductor on the electrically connected wafer grounded. In this case, charging is not a problem in practical use.
On the other hand, if the sample is non-conductive, or if the sample, even though made of a conductive material, is incapable of being grounded and thus is in an electrically floating state, there arises a problem that a charging state of the sample changes with time, so that a measurement target image drifts accordingly.
In a case, for example, where dimensions of a photomask used as an original plate for exposing a semiconductor are measured, charging occurs in the following two states. The first state is where a conductor such as chromium entirely lies on a glass substrate which is in the course of having wirings manufactured and where a resist wiring used for etching a wiring to chromium lies on the conductor. The second state is where the processing of manufacturing the wirings are completed so that a wiring made of the conductor such as chromium lies on the glass substrate.
Especially in the measurement after the etching process, most of the wiring patterns on the mask become incapable of being electrically grounded and thus are in an electrically floating state. In such a case, the surface potential of the measurement target wiring becomes extremely unstable.
This charging phenomenon changes with time and continues until the potential is stabilized. Therefore, after the potential is stabilized, a sample can be observed or measured stably. However, it takes a long time, several tens of seconds, for example, until the potential is finally stabilized. This leads to a problem of deterioration in throughput of measurement processing.